I personally like dual-type Water/Ground Pokemon because they're immune to Electric, resist Fire, Poison, Steel and Rock and are only weak to Grass, which is a type that's not seen very often in competitive battling (from my experiences, anyways). It depends on what your looking for for your team, though--Greninja, like the above poster said, is a Pokemon that very few Pokemon can switch in on successfully and is very offensive, but, if you're looking for some defensive sets, I'll list a couple of them that I personally like to use:
Swampert @ Chesto Berry
Ability: Torrent
EVs: 252 HP/4 Def/252 SpDef
Careful Nature
-Curse
-Earthquake
-Ice Punch/Avalanche
-Rest
(I put Avalanche as a suggestion as Ice Punch is a move that's only available through transfer from B2W2 and you may not have access to it. With the release of ORAS, you might be able to move tutor it, though. I, unfortunately, don't have Ice Punch on my Swampert :/)
Gastrodon @ Leftovers
Ability: Storm Drain
EVs: 252 HP/252 Def/4 SpDef
Bold/Relaxed Nature
-Scald
-Earth Power/Earthquake/Ice Beam
-Toxic
-Recover
(Again, I put suggestions for moves and natures. I got these from the Smogon site and, according to them, it's advised to run a Relaxed nature if you use Earthquake, while a Bold nature for Ice Beam or Earth Power. I run one with Ice Beam because I usually have several other Pokemon running Earthquake on them and I like having variety.)
I hope these help you in some way